Hi Will,
On Thu, Dec 10, 2015 at 05:39:59PM +0000, Will Deacon wrote:
> In paging_init, we allocate the zero page, memset it to zero and then
> point TTBR0 to it in order to avoid speculative fetches through the
> identity mapping.
>
> In order to guarantee that the freshly zeroed page is indeed visible to
> the page table walker, we need to execute a dsb instruction prior to
> writing the TTBR.
>
> Cc: <stable@xxxxxxxxxxxxxxx> # v3.14+, for older kernels need to drop the 'ishst'
> Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
> ---
> arch/arm64/mm/mmu.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index c04def90f3e4..c5bd5bca8e3d 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -464,6 +464,9 @@ void __init paging_init(void)
>
> empty_zero_page = virt_to_page(zero_page);
>
> + /* Ensure the zero page is visible to the page table walker */
> + dsb(ishst);
I think this should live in early_alloc (likewise in late_alloc).
In the other cases we call early_alloc or late_allot we assume the
zeroing is visible to the page table walker.
For example in in alloc_init_pte we do:
if (pmd_none(*pmd) || pmd_sect(*pmd)) {
pte = alloc(PTRS_PER_PTE * sizeof(pte_t));
if (pmd_sect(*pmd))
split_pmd(pmd, pte);
__pmd_populate(pmd, __pa(pte), PMD_TYPE_TABLE);
flush_tlb_all();
}
There's a dsb in __pmd_populate, but it's _after_ the write to the pmd
entry, so the walker might start walking the newly-allocated pte table
before the zeroing is visible.
Either we need a barrier after every alloc, or we fold the barrier into
the two allocation functions.
Thanks,
Mark.
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